Waxy feedstocks may be used to prepare basestocks having a high viscosity index (VI). However, in order to obtain a basestock having the low temperature properties suitable for most uses, it is usually necessary to dewax the feedstock. Dewaxing may be accomplished by means of a solvent or catalytically. Solvent dewaxing is a physical process whereby waxes are removed by contacting with a solvent, such as methyl ethyl ketone, followed by chilling to crystallize the wax and filtration to remove the wax. Catalytic dewaxing involves chemically converting the hydrocarbons leading to unfavorable low temperature properties to hydrocarbons having more favorable low temperature properties. Long chain normal paraffins and slightly branched paraffins readily solidify and thus result in generally unfavorable low temperature properties. Catalytic dewaxing is a process for converting these long chain normal paraffins and slightly branched paraffins to molecules having improved low temperature properties.
Catalytic dewaxing may be accomplished using catalysts that function primarily by cracking waxes to lower boiling products, or by catalysts that primarily isomerize waxes to more highly branched products. Catalysts that dewax by cracking decrease the yield of lubricating oils while increasing the yield of lower boiling distillates. Catalysts that isomerize do not normally result in significant boiling point conversion. Catalysts that dewax primarily by cracking are exemplified by the zeolites ZSM-5, ZSM-11, ZSM-12, beta and offretite. Catalysts that dewax primarily by isomerization are exemplified by the zeolites ZSM-22, ZSM-23, SSZ-32, ZSM-35, ZSM-48 and ZSM-50. To ensure adequate mechanical strength for use in a dewaxing reactor, such zeolite catalysts are generally combined with an inorganic oxide binder, such as alumina.
Conventional dewaxing catalysts are, however, susceptible to poisoning by sulfur and nitrogen contaminants in a feedstock. As a result, a hydrotreating step or other pre-treatment step often precedes a catalytic dewaxing step, in order to reduce the amount of sulfur and/or nitrogen in the feedstock. To mitigate the problem of catalyst poisoning and to allow effective dewaxing of feedstocks with very high levels of waxy materials, it is often desirable to be able to maximize the dewaxing activity of the catalyst. However, in seeking maximize activity, it is also important to maintain the mechanical strength of the catalyst.
In United States Published Patent Application No. 2009/0186754, one method of increasing the activity of a dewaxing catalyst has been identified, in which the activity improvement is achieved by using a zeolite with a low silica to alumina ratio in combination with a low surface area binder. The low surface area binder is believed to increase access to the active sites of the zeolite (e.g. acid sites). Especially for bulky feeds, increased access to zeolite active sites is expected to lead to an overall increase in activity.
However, tests have shown that the low surface area binder proposed in the '754 application tends to result in a catalyst with low crush strength and high particle density. Therefore, high solids extrusion mixtures are required to produce catalysts which achieve minimally acceptable crush strength which results in even higher particle density. Thus, if low surface area binders are to be employed to improve catalyst accessibility and activity, a method of improving the crush strength and reducing particle density of the catalyst is needed if the resultant catalyst is to be commercially viable.
According to the present disclosure, it has now been found that, by combining a zeolite with a low surface area binder (large crystallite size) mixed with a controlled and small amount of a high surface area binder (small crystallite size), it is possible to produce a catalyst having an attractive balance between catalytic activity and mechanical strength. Depending on the zeolite and operating conditions employed, the resultant catalyst can show improved performance as a dewaxing catalyst and in a variety of other catalytic processes.